Sneaking Around

After collecting the three passcodes in the beginning of chapter 4 you will come to the bridge area of the aircraft carrier. The relevant section for the trophy / achievement starts when Chris and Piers come up the elevator. Make sure that you have some non-explosive bolts for your crossbow before trying this. You have to kill at least 4 enemies, 2 guards and 2 "cockroaches". Don't try to sneak your way around the cockroaches. They would always notice you shortly before you drop out of the air vent.Stay out of the spotlights and only use the crossbow to take out enemies. When reaching the part with the second spotlight you have to perform a little slide to avoid being seen. If an enemy should see / hear you or even trigger an alarm then restart from the last checkpoint and try again.